In a lot of mobile communication standards, recently, CDMA cellular communication technique shows great potential for its features associated with large capacity, simple frequency planning, good communication quality and small electromagnetic interference. CDMA mobile communication has become the key technique of next generation of mobile communication systems.
Most of CDMA cellular mobile communication systems employ direct sequence spread spectrum (DSSS) technique. A transmitter converts the narrow band user information into wideband spread spectrum signals by multiplying narrow band information by a spreading code (PN code). This process is referred to as spread spectrum. A receiver restores the narrow band user information by multiplying received wideband spreading signals by the locally generated synchronous spreading code and integrating operation. This process is referred to as despreading operation. Accordingly, how to synchronizing local spreading code with the received signals is of importance to accomplish despreading operation in a receiver.
There is multipath fading in a mobile communication system, which causes serious multipath interference. In a CDMA cellular mobile communication system, the bandwidth of transmitting signals is typically much larger than that of user information because of the use of spread spectrum technique. Therefore, the CDMA cellular mobile communication system has the capability of distinguishing multipath. Therefore, it is possible to more finely distinguish multipath. However, since the multipath signals, which can be distinguished, have large radomicity, CDMA receivers should have the ability to overcome the multipath fading.
To synchronize with local spreading code in a CDMA receiver, the received signals need to be initially synchronized and finely synchronized. The initial synchronization process is also referred to as PN code acquisition process. The fine synchronization process is also referred to as PN code track process. The PN code initial synchronization process make the phase difference between the local PN sequence generated by the receiver and PN spreading sequence of the received signals smaller than a proper threshold δ. After the process of PN code initial synchronization, due to local oscillation frequency-offset, channel multipath fading, and Doppler frequency-offset etc., the timing will slide. Therefore, the received signals shall be tracked in real time so that the local PN sequence can finely synchronize with received signals all the time.
In CDMA cellular mobile communication systems, in addition to initially synchronize and track the cell which, a mobile terminal is located, it is necessary to search adjacent cells to judge whether or not the mobile terminal performs hand-off or macro-diversity receipt of the arrival signals of a plurality of base stations. The method of searching adjacent cells is similar with that of initial synchronization to a cell except for that the process of searching adjacent cells is only performed in partial area determined in advance without searching all phases of PN code.
CDMA systems have time restricts for initial synchronization of a cell and searching adjacent cells. The time for initial synchronization and searching adjacent cells shall be as short as possible. There are a lot of methods of initial synchronization and searching adjacent cells in DS/CDMA systems. These method may be classified into parallel initial synchronization, series initial synchronization and, parallel and series hybrid initial synchronization in terms of structure domain. In terms of physical domain, the method of initial synchronization may be classified into time domain initial synchronization, frequency domain initial synchronization. In terms of despreading methods, the method of initial synchronization may be classified into initial synchronization based on correlators and initial synchronization based on matching filter. The initial synchronization based on correlators includes fixed correlation length detection and variable correlation length sequential detection.
Conventional initial synchronization and search is used for a certain single-path signal. In multipath transmission circumstances, however, the real received signals typically comprise a plurality of multipath components which have different time-delay. The results of initial synchronization and search for a certain single-path signal are generally not reliable. The present invention is on the basis of the following fact: the energy window of multipath signal and the barycenter position of the window is relatively stable although the amplitude and phase of the signals for each arrival path randomly change in multipath circumstances. If the initial synchronization phase of the local PN code or the PN code phase of adjacent cells is determined based on maximizing multipath signal energy window, and the PN code is finely tracked based on barycenter change of the multipath energy window, the obtained spreading receiver can overcome the non-determinacy due to the processing of single path signals, thereby improving the stability of coherent spreading receivers.